Electric ceramic resistors having positive temperature coefficients and method of manufacturing same

ABSTRACT

A barium titanate semiconductive resistor having a high positive temperature coefficient of electrical resistance and containing a small amount of a fluoride.

United States Patent Yperman et al.

[ July 4, 1972 [541 ELECTRIC CERAMIC RESISTORS HAVING POSITIVETEMPERATURE COEFFICIENTS AND METHOD OF MANUFACTURING SAME [72]inventors: Jean Michel Baudry Ghislain yperman,

Charles Belhomme, Evere. both of Belgium [73] Assignee: U.S. PhilipsCorporation, New York. NY.

[22] Filed: Jan. 26, 1970 [21] Appl. No.: 6,052

Related US. Application Data [63] Continuation of Ser. No. 601,343, Dec.13, 1966,

abandoned.

[30] Foreign Application Priority Data Dec. 18, 1965 Netherlands..6516534 [52] US. Cl. ..252/520, 106/39 R, 252/521 [51] lnt.Cl -H0lc7/02 [58] Field of Search ..252/520,62.1. 106/39 R {56] References CitedUNITED STATES PATENTS 2,515,790 7/1950 Navias 106/63 2,981,699 4/1961lchikawa a ..252/520 Primary Examiner-Richard D. Lovering Attorney-FrankR. Trifari [57] ABSTRACT A barium titanate semiconductive resistorhaving a high positive temperature coefficient of electrical resistanceand containing a small amount of a fluoride.

8 Claims, No Drawings ELECTRIC CERAMIC RESISTORS HAVING POSITIVETEMPERATURE COEFFICIENTS AND METHOD OF MANUFACTURING SAME Thisapplication is a continuation of application Ser. No. 601,343, filedDec. 13, 1966 and now abandoned.

The invention relates to a method of manufacturing ceramic resistancebodies for electric resistors having a positive temperature coefficientof the electric resistivity, which bodies mainly consist ofsemiconductive barium titanate; the invention further relates toresistance bodies thus manufactured and resistors manufactured by meansof said bodies (so-called FTC-resistors).

The expression semiconductor ceramic resistance bodies on the basis ofbarium titanate is to be understood to mean hereinafter such bodieswhich have been made semiconductive with the use of the principle of thecontrolled valency.

FTC-resistors the resistance body of which consists of semiconductivebarium titanate, the electric resistance of which can increaseconsiderably when the temperature increases to and above theferro-electric Curie-temperature are known per se. Examples hereof arethose resistors the ceramic resistance bodies of which consist of bariumtitanate in which a part of the barium ions is replaced by la ions or Sbions or part of the Ti ions by Nb ions.

Applicants have found that PlC-resistors can be manufactured the saidtemperature-dependence of the electric resistance of which isconsiderably larger than of known PTC- resistors on the basis ofsemiconductive titanate.

PTCresistors manufactured according to the invention show a considerablystronger increase of the electric resistance than the known resistors onthe basis of semiconductive barium titanate when the temperature isincreased to and above the ferro-electric Curie-temperature, or theabsolute increase of the electric resistance in a given temperaturerange is considerably larger than of those known resistors; often botheffects occur in a FTC-resistor obtained with the use of the invention.These FTC-resistors often have a substantially constant temperaturecoefficient of the electric resistance over a large temperature range.

Consequently, the FTC-resistors according to the invention canadvantageously be used in those cases where electric resistors havingpositive temperature coefficients can be used, for example, for currentlimiting and stabilization in thermoregulators and voltage stabilizers,for safeguarding electric motors and for temperature and radiationmeasurements.

The invention relates to a method of manufacturing a ceramic resistancebody consisting of substantially pure semiconductive barium titanatewhich has been made semiconductive with the use of the principle of thecontrolled valency from a powder which substantially consists of bariumtitanate or substantially of compounds which are converted into bariumtitanate upon heating in an oxygen-containing atmosphere, characterizedin that before shaping the powder is intimately mixed with a fluoride ofan element of the group consisting of Ca, Sr and Ba in a quantitycorresponding to 0.5 at. percent of fluorine, calculated on bariumtitanate present in the ceramic resistance body, to a ceramic resistancebody thus manufactured and PTC-resistors manufactured by means of saidbody. 0

Applicants have found that for the manufacture of PTC-resistorsaccording to the invention the ceramic resistance bodies thereof mustconsist of substantially pure semiconductive barium titanate andpreferably of such a barium titanate which contains a small excess, upto 3 mol. percent and preferably 1.5 2.0 mol. percent, of titaniumdioxide.

A ceramic resistance body consisting of substantially puresemiconductive barium titanate is to be understood to mean herein such abody which consists for at least 95 mol. percent of barium titanate.

For making barium titanate semiconductive the known methods may be usedand at least one element which introduces semiconductivity with the useof the principle of the controlled valency may be incorporated insuitable quantifies. Examples of such elements are antimony, bismuth,niobium, cerium, yttrium, rare earth metals, for example, lanthanum.

As a fluoride is preferably used calcium fluoride.

The quantity of fluoride to be used corresponds to 0.5 10 at. percent offluorine, calculated on barium titanate in the ceramic resistance body.This content preferably corresponds to l 8 at. percent of fluorine andin particular to 2 6 at. percent of fluorine.

The manufacture of ceramic resistance bodies according to the inventionmay be carried out, for example, as follows:

Powders of barium carbonate, titanium dioxide and antimony oxide, (Sb Oare carefully mixed, for example, by grinding, and the mixture is heatedin air at l,l00 C for 60 minutes. The resulting product is ground. Thefluoride to be added, for example, Calis intimately mixed with theresulting powder, for example, by grinding. The resulting pulverulentproduct is given the desired shape for example, of a rod, sheet orstrip, for example, by compression. The body thus obtained is sinteredin an oxygen-containing atmosphere to form a ceramic resistance body,for example, by heating in air at l,400 C for 60 minutes. The resultingresistance body may be provided in known manner with ohmic electrodesand supply wires for the manufacture of PT C-resistors.

Ceramic resistance bodies to be investigated (wafers, diameter 7.5 mms,thickness 1.85 mms) were provided with ohmic electrodes as a result ofwhich no contact resistances occur. The electric resistance was measuredat different temperatures.

In the following table are successively stated the number of the test;the nature and the quantity of fluoride added; iculars, if any,regarding sintering temperature, sintering uration and replacement ofpart of BaO by SrO or of TiO by SnO respectively; and finally themaximum value of the temperature coefficient of the electric resistivitydenoted by TC,,,,,,, in per cent per C.

The basic composition of the ceramic bodies was the followmg:

d" log R r H dT The increase in the electric resistivity at a rise intemperature of 1 C is (T, temperature in C.)

(e= base of natural logarythms.)

For a TC-value of mo percent per C, e" 2.78 which means that per C theelectric resistance in this case increases by a factor 2.78.

It will be clear that, since with the use of the method according to theinvention ceramic resistance bodies can be manufactured which havetemperature coefficients of the electric resistivity which are higherthan percent per C, the increase of the electric resistance at a rise intemperature of 1 C is particularly large in FTC-resistors manufacturefrom said bodies.

The ceramic bodies according to the present invention are of a finecrystallinity. Applicants suppose that this is due to a limitation ofcrystal growth by the influence of fluoride. Due to this the bodies showa high breakdown voltage. The high order of reproducibility in themanufacture of the resistors is also due to the said effect.

TABLE Test Nr. Fluoride Particulars TC in in mol per C B O 38 l0.3%;CaF, 51

2 1.0%;CaF 1 l5 3 10%;CaF 5i 4 1% ;CaF sinterlng l400C 21 min. 5 as 4sintering 1400C 92 40 min. 6 as 4 sintering l400C 1 I5 80 min. 7 l CaFsintering at 1350C 92 for 40 min. 8 as 7 ditto 1400C 92 9 as 7 ditto1500C 78 l0 1 CaF- 2 mol of BaO 92 replaced by SrO 1 1 1 ;CaF 2 molofTiO 58 replaced by SnO, r2 1 Bar 65 13 l SrF, 2 mol of BaO 68 replacedby SrO We claim:

1. In an electric ceramic resistor having a positive temperaturecoefficient of electrical resistance and comprising a barium titanatebody and a metal capable of inducing semiconductivity into said bariumtitanate and provided with at least one ohmic contact electrode, theimprovement wherein said semi-conductive body contains 0.5 to at.percent inclusive of fluorine in the form of barium, calcium orstrontium fluoride calculated on the barium titanate.

2. The electric ceramic resistor of claim 1 wherein said semiconductorbody contains 1 to 8 at. percent of fluorine in the form of fluoridebased on the barium titanate.

3. The electric ceramic resistor of claim 1 wherein the ceramic bodycontains 2 to 6 at. percent inclusive of fluoride in the form offluoride calculated on the barium titanate.

4. The electric ceramic resistor of claim 1 wherein the ceramic bodycontains 1.5 2 mol. percent of titanium dioxide calculated on the bariumtitanate.

5. In the method of manufacturing a ceramic resistance body consistingof semiconductive barium titanate comprising the steps, first shaping apowder consisting substantially of barium-titanate or compoundsheat-convertable to barium titanate and a compound of a metal capable ofinducing semiconductivity into the barium titanate into a shaped massand then sintering the resultant shaped mass by heating in oxygen, theimprovement which comprises adding to the powder before shaping at leastone fluoride of an element selected from the group consisting of Ca, Srand Ba in a quantity corresponding to 0.5-10 at. percent of fluorine,calculated on the barium titanate present in the ceramic resistancebody.

6. The method of claim 5 wherein the fluoride is employed in an amountcorresponding to l-8 at. percent of fluorine, calculated on the bariumtitanate present in the ceramic resistance body.

7. The method of claim 5 wherein the fluoride is employed in an amountcorresponding to 2-6 at. percent of fluorine calculated on the bariumtitanate present in the ceramic resistance body.

8. The method of claim 5 wherein the fluoride is a calcium fluoride.

75g UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3674 713 Dated July 4,1972

Inventor(s) JEAN MICHEL BAUDRY GHISLAIN YPERMAN ET AL It is certifiedthat error appears in the above-identified patent and that said LettersPatent are hereby corrected as shown below:

I In the title page:

[ "yperman" should read Yperman "Everefl'should be cancelled.

Signed and sealed this 9 day of January 1972.

(SEAL) Attest:

EDWARD M.FLETCHER,JR. ROBERT GOTTSCHALK Attesting Officer Commissionerof Patents

2. The electric ceramic resistor of claim 1 wherein said semiconductorbody contains 1 to 8 at. percent of fluorine in the form of fluoridebased on the barium titanate.
 3. The electric ceramic resistor of claim1 wherein the ceramic body contains 2 to 6 at. percent inclusive offluoride in the form of fluoride calculated on the barium titanate. 4.The electric ceramic resistor of claim 1 wherein the ceramic bodycontains 1.5 - 2 mol. percent of titanium dioxide calculated on thebarium titanate.
 5. In the method of manufacturing a ceramic resistancebody consisting of semiconductive barium titanate comprising the steps,first shaping a powder consisting substantially of barium-titanate orcompounds heat-convertable to barium titanate and a compound of a metalcapable of inducing semiconductivity into the barium titanate into ashaped mass and then sintering the resultant shaped mass by heating inoxygen, the improvement which comprises adding to the powder beforeshaping at least one fluoride of an element selected from the groupconsisting of Ca, Sr and Ba in a quantity corresponding to 0.5-10 at.percent of fluorine, calculated on the barium titanate present in theceramic resistance body.
 6. The method of claim 5 wherein the fluorideis employed in an amount corresponding to 1-8 at. percent of fluorine,calculated on the barium titanate present in the ceramic resistancebody.
 7. The method of claim 5 wherein the fluoride is employed in anamount correSponding to 2-6 at. percent of fluorine, calculated on thebarium titanate present in the ceramic resistance body.
 8. The method ofclaim 5 wherein the fluoride is a calcium fluoride.